Semiconductor lasers are now almost in the stage of practical use as light sources for optical communication. Especially, in the neighborhood of a wavelength of 1.3 .mu.m, fibers of the silica system have an extremely low loss. This has led to active development of InGaAsP/InP semiconductor lasers capable of obtaining laser light with the above wavelength. Use of single mode fibers is particularly useful for communication in the band of this wavelength.
The light source for the single mode fiber requires the following characteristics:
(1) transverse single mode and spot-like oscillation, PA1 (2) small beam divergence, PA1 (3) small operating current, etc.
The conventional, especially primitive, lasers of the stripe type are liable to cause multimode oscillation as the operation current is increased. Furthermore, the operation current is extremely large. In order to attain a low threshold current and mode stability, a buried-type laser has been proposed. As shown in FIG. 1, an n-InP layer 2, an n-InGaAsP active layer 3, a p-InP layer 4 and a p-InGaAsP cap layer 5 are successively grown on an n-InP substrate 1 by the first liquid phase epitaxial growth. These layers are processed by etching in an inverted mesa shape. Thereafter, a current block layer composed of a p-InP layer 6 and an n-InP layer 7 is grown. In such a structure, since the width of the active layer is about 2 .mu.m and the whole region surrounding the layer is made of InP with a small refractive index, light and current are confined efficiently. Thus, transverse single mode oscillation is obtained at a low threshold current.
However, a difficulty in manufacturing the above structure is that unless the width of the active layer formed by etching after the first epitaxial growth is of the order of 2 .mu.m, no transverse single mode oscillation can be obtained.
In the present stage, wet-etching has been the only method for obtaining such a narrow etching. However, this method has poor controllability. Namely, only a slightly fast progress of etching causes breaking of the active layer. On the other hand, if the width of the active layer is made too large, no transverse single mode oscillation is obtained although the active layer is not easily broken. The most critical problem of the conventional method is that unless the width of active layer is equal to or less than 3 .mu.m, when its thickness is assumed to be about 0.2 .mu.m, no transverse single mode is realized. As described above, the fact that the laser oscillates in the single mode is the essential condition for making the best use of the advantage of the single mode fiber. Therefore, the controllability of etching may be considered to be the most important problem of the lasers of the above-mentioned type.
This invention relates to a semiconductor laser which is used as a light emitting element in the field of optical communication and related fields.